Common mode voltage feed fault protection

ABSTRACT

A system includes a generator. Three AC feeders are connected for feeding AC output from the generator. A rectifier is electrically connected to the three AC feeders and to a load via a first DC feeder and a second DC feeder. A first resistor connects between a first one of the DC feeders and ground. A first voltage sensor is operatively connected to detect voltage across the first resistor. A second resistor connects between the second DC feeder and ground. A second voltage sensor is operatively connected to detect voltage across the second resistor. A controller is configured to monitor for changes in common mode voltage based on the input from the first sensor and from the second sensor, and to determine presence of a fault if change in the common mode voltage exceeds a predetermined threshold.

BACKGROUND 1. Field

The present disclosure relates to ground fault protection for electricalsystems, and more particularly to fault protection for systems such asaircraft.

2. Description of Related Art

Traditional constant frequency or variable frequency generation systemson aircraft have a neutral connection which is brought out of thegenerator and tied to ground. If an electrical feeder in thedistribution system became shorted to ground, large amounts of currentwould flow back to the generator through the airframe. This type ofhistorical electrical system grounding topology enabled adequate currentsourcing levels and an provided associated current sensing functionalityfor the fault to be detected and isolated by generation/distributionequipment controllers.

To reduce aircraft weight on future applications, electrical systemdesigns are continuing to expand utilization of high voltage DCgeneration and distribution at +/−270 Vdc or +/−540 Vdc. With an +/−270Vdc, +/−540 Vdc or equivalent system, beyond traditional generatorneutral grounding, an impedance grounding scheme topology optioninterfaced at the system rectifier output midpoint is feasible. Thistype of impedance grounding approach is an important factor whencompared to traditional generator neutral grounding, impactingelectrical system performance characteristics and overcurrent protectioncoordination during feeder/load equipment ground faults.

For ground schemes that have the generator neutral floating with respectto ground, and the DC link of the rectifier is connected to groundthrough a resistor, which has some distinct advantages as a groundingapproach. Namely, a fault of either an AC or DC aircraft feeder to theairframe will significantly reduce fault sourcing current levels anddoes not appreciably affect the DC output performance, allowing thesystem to continue to operate if desired. Reduced fault current with ahigh impedance ground fault condition has benefit for high voltage DCsystems to mitigate potential equipment damage. However, since largeamounts of current no longer flow during the fault condition,traditional current based feeder fault detection methods are no longerviable with this impedance grounding topology, so new methods need to bedeveloped for fault isolation coordination.

Conventional generator neutral grounded systems and associatedovercurrent fault detection techniques have been considered satisfactoryfor their intended purpose. However, there is a need for improved groundfault detection for power systems aboard aircraft utilizing high voltageDC generation/distribution.

SUMMARY

A system includes a generator. Three AC feeders are connected forfeeding AC output from the generator. A rectifier is electricallyconnected to the three AC feeders. The rectifier is configured toconvert three phase AC from the three AC feeders of the generator intoDC output to a load via a first DC feeder and a second DC feeder. Afirst resistor connects between a first one of the DC feeders andground. A first voltage sensor is operatively connected to detectvoltage across the first resistor. A second resistor connects betweenthe second DC feeder and ground. A second voltage sensor is operativelyconnected to detect voltage across the second resistor. A controller isoperatively connected to receive input from the first voltage sensor andfrom the second voltage sensor. The controller is configured to monitorfor changes in common mode voltage based on the input from the firstsensor and from the second sensor, and to determine presence of a faultif change in the common mode voltage exceeds a predetermined threshold.

The controller can be configured to determine presence of a fault ifchange in value of the common mode voltage exceeds a predeterminedthreshold. The controller can be configured to determine the fault islocated in one of the DC feeders if the value of the common mode voltageis constant at a level beyond a predetermined threshold. The controllercan be configured to isolate the fault to a positive DC feeder if thecommon mode voltage is negative, and to isolate the fault to a negativeDC feeder if the common mode voltage is positive. The controller can beconfigured to determine the fault is located in one of the AC feeders ifthe value of the common mode voltage follows a waveform. The controllercan be configured to determine the in which one of the AC feeders thefault is located based on phase angle of the value of the common modevoltage matching phase angle sensed in the AC feeder with the fault.

A DC filter can be operatively connected to each of the first and secondDC feeders, and to ground, wherein the DC filter is configured to reducenoise in the first and second DC feeders. A DC load can be operativelyconnected to each of the first and second DC feeders, wherein the loadis configured to utilized DC power from the first and second DC feeders.A second set of three AC feeders can connect a second three phases ofthe AC generator to a second rectifier. The second rectifier can includetwo DC feeders operatively connected to the two DC feeders of the firstrectifier. A second generator and at least one respective rectifier ofthe second generator can be included, wherein two DC feeders of the atleast one respective rectifier of the second generator are connected inparallel to the first and second DC feeders, respectively, of the firstrectifier. The controller can be part of a generator control unit (GCU)or rectifier operatively connected to control the generator, part of amotor control unit (MCU) connected to control a motor connected to bepowered as a load by the first and second DC feeders, or a separatecontroller from the GCU, rectifier, and MCU. The controller can beconfigured to take corrective action upon detection of a fault. Thecontroller can be configured to de-excite the generator, disconnect themotor from power, or provide an alternative source of power to the loadas the corrective action.

A method includes detecting a first voltage across a first resistorbetween a first DC feeder from a rectifier and ground, detecting asecond voltage across a second resistor between a second DC feeder fromthe rectifier and ground, and using the first and second voltages tocalculate a common mode voltage. The method includes determining thecommon mode voltage, and determining a fault has occurred base on achange in the common mode voltage that exceeds a predeterminedthreshold.

The method can include determining the fault is located in one of the DCfeeders if the common mode voltage is constant, and if the fault islocated in one of the DC feeders: isolating the fault to a positive DCfeeder if the common mode voltage is negative; and isolating the faultto a negative DC feeder if the common mode voltage is positive. Themethod can include determining the fault is located in one of the ACfeeders if the common mode voltage follows an AC waveform, and if thefault is located in one of the AC feeders: determining which one of theAC feeders has the fault wherein phase angle of the common mode voltagematches phase angle sensed in one of the AC feeder that has the fault.

These and other features of the systems and methods of the subjectdisclosure will become more readily apparent to those skilled in the artfrom the following detailed description of the preferred embodimentstaken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject disclosureappertains will readily understand how to make and use the devices andmethods of the subject disclosure without undue experimentation,preferred embodiments thereof will be described in detail herein belowwith reference to certain figures, wherein:

FIG. 1 is a schematic view of an embodiment of a system constructed inaccordance with the present disclosure, showing the generators,rectifiers with high impedance ground, feeders, and load equipmentinterface with motor controller/motor and SSPC (solid state powercontroller) device distribution;

FIG. 2 is a graph showing a DC feeder fault apparent in the common modevoltage for the system of FIG. 1 ; and

FIG. 3 is a graph showing an AC feeder fault apparent in the common modevoltage for the system of FIG. 1 .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like referencenumerals identify similar structural features or aspects of the subjectdisclosure. For purposes of explanation and illustration, and notlimitation, a partial view of an embodiment of a system in accordancewith the disclosure is shown in FIG. 1 and is designated generally byreference character 100. Other embodiments of systems in accordance withthe disclosure, or aspects thereof, are provided in FIGS. 2-3 , as willbe described. The systems and methods described herein can be used todetect AC feeder faults, DC feeder faults, and distribution groundfaults using common mode voltage.

The system 100 includes a generator 102. Three AC feeders 104 a, 104 b,104 c are connected for feeding AC output from the generator 102. Arectifier 106 is electrically connected to the three AC feeders 104 a,104 b, 104 c. The rectifier 106 is configured to convert three phase ACfrom the three AC feeders 104 a, 104 b, 104 c of the generator 102 intoDC output to a load via a first DC feeder 108 p and a second DC feeder108 n. A first resistor 110 connects between the DC feeder 108 p andground 112. A first voltage sensor 114 is operatively connected todetect voltage across the first resistor 110. A second resistor 116connects between the second DC feeder 108 n and ground 112. A secondvoltage sensor 118 is operatively connected to detect voltage across thesecond resistor 116. A DC filter 120 is operatively connected to each ofthe first and second DC feeders 108 p, 108 n, and to ground 112, whereinthe DC filter 120 is configured to reduce noise in the first and secondDC feeders 108 p, 108 n. The resistors 110 and 116 with voltage sensors114 and 118 could be integrated as part of an associated channelRectifier/DC Filter combined unit.

DC load(s) 122 is/are operatively connected to each of the first andsecond DC feeders 108 p, 108 n, wherein the load 122 is configured toutilized DC power from the first and second DC feeders 108 p, 108 n andcan host common mode voltage sense functionality 122 a (with sensors oneach feeder or as a single sensor coupling each feeder). DC load(s) canalso be interfaced with SSPC (solid state power controller)+/−270 Vdcdevices 200, which include integrated common mode voltage sensefunctionality 200 a (e.g. with sensors on each feeder or as a singlesensor coupling each feeder). Additional load equipment interface mayinclude, motor controller/motor equipment 123, also including integratedcommon mode voltage sense functionality 123 a (e.g. with sensors on eachfeeder or as a single sensor coupling each feeder).

A controller 124 is operatively connected to receive input from thefirst voltage sensor 114 and from the second voltage sensor 118. Thecontroller 124 is configured to monitor for changes in common modevoltage based on the input from the first and second sensors 114, 118,and to determine presence of a DC feeder to ground fault 126, or ACfeeder to ground fault 128, if change in the common mode voltage exceedsa predetermined threshold. The common mode voltage is the sum of thevoltage measurements from the sensors 114, 118 as indicated in FIGS. 2-3.

The controller 124 is configured to determine presence of a fault ifchange in value of the common mode voltage exceeds a predeterminedthreshold. The controller 124 can determine the fault 126 is located inone of the DC feeders 108 p, 108 n if the common mode voltage isconstant at a level beyond a predetermined threshold, i.e. always on thesame side of zero with sufficient magnitude in the common mode (Vdc CM)graph in FIG. 2 . This type of fault is indicated schematically in FIG.1 with the fault 126, which is shown in the feeder 108 p, but could bein the feeder 108 n as well. The controller is configured to isolate thefault to a positive DC feeder (feeder 108 p) if the common mode voltageis negative (FIG. 2 shows negative Vdc CM value for a positive DC feederto ground fault), and to isolate the fault to a negative DC feeder(feeder 108 n) if the common mode voltage is positive.

The controller 124 is configured to determine the fault 128 is locatedin one of the AC feeders 104 a, 104 b, 104 c if the common mode voltagefollows a waveform, i.e. increases in magnitude with crosses over zeroas in the common mode (Vdc CM) graph in FIG. 3 . The fault 128 is shownschematically in FIG. 1 in the feeder 104 a, but it could be in theother two AC feeders 104 b, 104 c as well. The controller 124 candetermine the in which one of the AC feeders 104 a, 104 b, 104 c thefault 128 is located based on phase angle of the common mode voltagematching phase angle sensed in the AC feeder 104 a, 104 b, 104 c withthe fault 128.

With ongoing reference to FIG. 1 , any suitable number of generators,feeders, rectifiers, filters can be included. A second set of three ACfeeders 104 x, 104 y, 104 z connect a second three phases of the ACgenerator 102 to a second rectifier 130. The second rectifier 130includes two DC feeders 108 p, 108 n operatively connected to the two DCfeeders of the first rectifier 106, i.e. the feeders 108 p and 108 n areconnected in parallel for the two rectifiers 106 and 130, which canshare the same DC filter 120.

A second generator 132 and at least respective rectifiers 134, 136 ofthe second generator 132 can be included, wherein two DC feeders 108 p,108 n of the at rectifiers 134, 136 of the second generator 132 areconnected to the first and second DC feeders, respectively, of the firstrectifier 106, 130, i.e. the respective DC feeders 108 p, 108 n are allconnected in parallel for all four rectifiers 106, 130, 134, 136. Thesix respective AC feeders of the second generator 132 are not numbered,for sake of clarity in the drawing, but are connected to respectiverectifiers 134, 136 in the same manner described above for generator102. The rectifiers 134, 136 are operatively connected to their own DCfilter 138 in the same manner described above for rectifiers 106, 130.There can be additional resistors 140, 142 connected to ground 112, withrespective voltage sensors 144, 146 all connected to the feeders 108 p,108 n, and to the controller 124 in the same manner as for thoseresistors 110, 116 described above. Also, the resistors 140 and 142 withvoltage sensors 144 and 146 could be integrated as part of an associatedchannel Rectifier/DC Filter combined unit. Each of the generators 102,132 can have a respective POR (point of regulation) 148 between itsrespective DC filter 120, 138 and the load 122. For example, the GCU(generator control unit) can regulate the POR 148 to +/−270 Vdc (540 Vdcacross feeders), but other voltage levels are conceivable, depending onapplication, such as +/−540 (1080 Vdc across feeders).

The controller 124 can be a separate unit, or can be part of one or morea generator control units (GCUs) 150 operatively connected to controlthe generators 102, 132. It is also contemplated that the controller 124can be part of rectifiers 106, 130, 134, 136 or a motor control unit(MCU) 123 connected to control a motor connected to be powered as a load122 by the first and second DC feeders 108 p,108 n. A controller 124 canalso be part of SSPC (solid state power controller) devices 200 forcommon mode detection in the distribution system. The controller 124 isconfigured to take corrective action upon detection of a fault 126, 128.For example, the controller 124 can de-excite the affected the generator102, 132, disconnect the motor (e.g. in load 122) from power, provide analternative source of power to the load 122 as the corrective action,alert other systems or users of the fault, or the like.

Systems and methods as disclosed herein provide potential benefits forhigh impedance ground applications such as good system power qualityperformance, no significant common mode (CM) voltage during normaloperation, quick recovery from system fault, and current does not flowto ground in the presence of a single fault to ground—allowing forpotential continued operation. Systems and methods as disclosed hereinprovide potential benefits for common mode (CM) voltage based feederfault detection including utilizing existing sensing for control,potentially eliminating dedicated current sensors—lowering aircraftweight and cost, the ability to isolate faults to different feedersbased on CM voltage signature giving some ability to isolate faultswithout staged timing, and potentially using simple average/RMScalculations for fault detection—with no complex algorithms required.

The methods and systems of the present disclosure, as described aboveand shown in the drawings, provide for detection of AC feeder faults andDC feeder faults using common mode voltage. While the apparatus andmethods of the subject disclosure have been shown and described withreference to preferred embodiments, those skilled in the art willreadily appreciate that changes and/or modifications may be made theretowithout departing from the scope of the subject disclosure.

What is claimed is:
 1. A system comprising: a generator; three ACfeeders connected for feeding AC output from the generator; a rectifierelectrically connected to the three AC feeders, the rectifier beingconfigured to convert three phase AC from the three AC feeders of thegenerator into DC output to a load via a first DC feeder and a second DCfeeder; a first resistor connecting between a the first DC feeder andground; a first voltage sensor operatively connected to detect voltageacross the first resistor; a second resistor connecting between thesecond DC feeder and ground; a second voltage sensor operativelyconnected to detect voltage across the second resistor; and a controlleroperatively connected to receive input from the first voltage sensor andfrom the second voltage sensor, wherein the controller is configured tomonitor for changes in common mode voltage based on the input from thefirst sensor and from the second sensor, and to determine presence of afault if change in the common mode voltage exceeds a predeterminedthreshold, wherein the controller is configured to determine presence ofa fault if change in value of the common mode voltage exceeds apredetermined threshold, wherein the controller is configured to isolatethe fault to a positive DC feeder if the common mode voltage isnegative, and to isolate the fault to a negative DC feeder if the commonmode voltage is positive.
 2. The system as recited in claim 1, whereinthe controller is configured to determine the fault is located in one ofthe DC feeders if the common mode voltage is constant at a level beyonda predetermined threshold.
 3. The system as recited in claim 1, furthercomprising a DC filter operatively connected to each of the first andsecond DC feeders, and to ground, wherein the DC filter is configured toreduce noise in the first and second DC feeders.
 4. The system asrecited in claim 1, further comprising a DC load operatively connectedto each of the first and second DC feeders, wherein the load isconfigured to utilized DC power from the first and second DC feeders. 5.The system as recited in claim 1, wherein the three AC feeders are afirst set of AC feeders connecting a first three phases of the ACgenerator to the rectifier, wherein the rectifier is a first rectifierand further comprising: a second set of three AC feeders connecting asecond three phases of the AC generator to a second rectifier, whereinthe second rectifier includes two DC feeders operatively connected tothe two DC feeders of the first rectifier.
 6. The system as recited inclaim 5, further comprising a second generator and at least onerespective rectifier of the second generator, wherein two DC feeders ofthe at least one respective rectifier of the second generator areconnected in parallel to the first and second DC feeders, respectively,of the first rectifier.
 7. The system as recited in claim 1, wherein thecontroller is part of a generator control unit (GCU) or rectifieroperatively connected to control the generator, part of a motor controlunit (MCU) connected to control a motor connected to be powered as aload by the first and second DC feeders, or a separate controller fromthe GCU, rectifier, and MCU.
 8. The system as recited in claim 7,wherein the controller is configured to take corrective action upondetection of a fault.
 9. The system as recited in claim 7, wherein thecontroller is configured to de-excite the generator, disconnect themotor from power, or provide an alternative source of power to the loadas the corrective action.
 10. A system comprising: a generator; three ACfeeders connected for feeding AC output from the generator; a rectifierelectrically connected to the three AC feeders, the rectifier beingconfigured to convert three phase AC from the three AC feeders of thegenerator into DC output to a load via a first DC feeder and a second DCfeeder; a first resistor connecting between a the first DC feeder andground; a first voltage sensor operatively connected to detect voltageacross the first resistor; a second resistor connecting between thesecond DC feeder and ground; a second voltage sensor operativelyconnected to detect voltage across the second resistor; and a controlleroperatively connected to receive input from the first voltage sensor andfrom the second voltage sensor, wherein the controller is configured tomonitor for changes in common mode voltage based on the input from thefirst sensor and from the second sensor, and to determine presence of afault if change in the common mode voltage exceeds a predeterminedthreshold, wherein the controller is configured to determine presence ofa fault if change in value of the common mode voltage exceeds apredetermined threshold, wherein the controller is configured todetermine the fault is located in one of the AC feeders if the commonmode voltage follows an AC waveform.
 11. The system as recited in claim10, wherein the controller is configured to determine the in which oneof the AC feeders the fault is located based on phase angle of the valueof the common mode voltage matching phase angle sensed in the AC feederwith the fault.
 12. A method comprising: detecting a first voltageacross a first resistor between a first DC feeder from a rectifier andground; detecting a second voltage across a second resistor between asecond DC feeder from the rectifier and ground; using the first andsecond voltages to calculate a common mode voltage; determining thecommon mode voltage; determining a fault has occurred base on a changein the common mode voltage that exceeds a predetermined threshold;determining the fault is located in one of the DC feeders if the commonmode voltage is constant, and if the fault is located in one of the DCfeeders: isolating the fault to a positive DC feeder if the common modevoltage is negative; and isolating the fault to a negative DC feeder ifthe common mode voltage is positive; and determining the fault islocated in one of the AC feeders if the common mode voltage follows anAC waveform, and if the fault is located in one of the AC feeders:determining which one of the AC feeders has the fault wherein phaseangle of the common mode voltage matches phase angle sensed in one ofthe AC feeder that has the fault.